Possible, but not so likely with directly heated valves. There are often good reasons for each triode having its own independent filament supply.
Yes, but there should be a valve in the socket otherwise the supply voltage is likely to run high (depending on whether it’s regulated). Then making a measurement will involve turning the amp on its side without risking any physical pressure on the 300Bs and, if possible, keeping the filament plane vertical so gravity doesn’t pull the hot metal towards the grid. Then it’s just a matter of inserting some test prods while staying well away from the 400V HT. All this gets easier/safer once you’ve done it a few times, but there has to be a first time for everyone who’s ever going to do it.
When I worked at the lab we used to say that in many trades one of the best ways to learn was ‘from your mistakes’. In HV engineering though that tended to be ‘from other people’s mistakes’. You could read about those in the coroners’ reports .
If you suspect that a filament is open-circuit you can sometimes confirm that by having a very close look at it where it’s attached to the top and bottom micas. The filament will likely zig-zag up and down from one mica to the other, quite possibly being tensioned by little springs at the micas. It may have one electrical contact in the middle (probably at the bottom mica) and two more - one at each end. The end contacts can be wired together down to one of the base ‘filament’ pins with the other base filament pin connected to the centre of the filament.
The reason I’m mentioning all this detail is that if a centre-tapped filament itself should snap then that can cause only one half of the filament to go dark. If, on the other hand, both halves of the filament go dark then that points towards the problem being in the wiring between the filament and the base pins.
In old valves it wasn’t unusual for the connection between the wiring and the base pin to fail outside the glass vacuum envelope (i.e. inside the base). With care, faults like that can sometimes be repaired, unlike any faults inside the glass which, obviously, can’t.
I’m just wondering if there’s any chance the filament fault in the valve could be a disconnection inside the valve’s base (I don’t mean the socket that it sits in) but outside the glass. It’s not very likely, I fear, but if the connection could be re-made then that might save a few quid.
This pic might help (it’s not a 300B, obviously)
On the left is the glass vacuum envelope with the individual wires coming out. They’re poked down the hollow pins of the base (on the right) and then solder is run up the inside of each pin to make a connection to the wire. Sometimes that solder joint can be faulty when made. Sometimes it can fail with age. Then the valve will behave as ‘failed’ but it is, in principle at least, repairable.
The tube is only just over a year old Graeme and (unlike the one in your pic) still looks brand new.
I’ve just dropped Definitive a message explaining what’s happened and I shall follow up with a call later. Fingers crossed, despite it being a few days outside the warranty period, they will be helpful.
Actually Jon, it only been just shy of two years since the first time I bought kit from them (the Gotos)
The combined value of the Gotos and Taks might seem like a lot to some folk, but I suspect by Definitive’s standards, it’s probably chicken feed.
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